RE: [PATCH] libnvdimm/dimm: Add a no-BLK quirk based on NVDIMM family

[Dexuan Cui]

> From: Dan Williams <dan.j.williams@xxxxxxxxx>
> Sent: Sunday, February 3, 2019 9:49 AM
> > ...
> > It looks the namespace created by Ubuntu 19.04 (4.18) is incompatible with
> > the libnvdimm-pending branch + this patch.
> 
> This is correct, the configuration switched from label-less by default
> to labeled.
Thanks for the confirmation! 
 
> > It looks we need to completely disable the label usage for
> NVDIMM_FAMILY_HYPERV
> > due to compability?
> 
> It's either going to be a quirk in Linux or a quirk in the Hyper-V
> configuration. I think it's more manageable for Hyper-V to ship a
> "label disable" switch than to try to ship a workaround in Linux. The
> "noblk" quirk in Linux works around the LOCAL bit in the labels.
> However, the namespace mode regression can only be resolved by hiding
> the label capability until the administrator manually switches from
> label-less to labeled operation.

As I understand, the essence of the issue is: Hyper-V emulates the
label mechanism (i.e. it supports _LSI and LSR), but doesn't do it
right (i.e. it doesn't support _LSW).

To manage the namespaces, Linux can choose to use label or not.

If _LSI/_LSR are supported, Linux assumes _LSW is supported as well
and chooses to use label (i.e. the label mode), but since Hyper-V doesn't
support _LSW, Linux fails to change the namespace configuration. In
Ubuntu 19.04 (4.18), the kernel is not aware of Hyper-V _LSI/_LSR, so
the created namespace is in "label-less" mode, and hence can't be used
with the libnvdimm-pending branch + this patch, unless we add a quirk
in Linux to explicitly not use the label.

I agree ideally Hyper-V should hide the label capability, and I'll request
Hyper-V team to do that. I hope Hyper-V guys are still able to do that
in time so we won't need a quirk in Linux kernel.

BTW, I suppose Windows VM should be in "label-less" mode.

Thanks for the help, Dan!

Thanks,
-- Dexuan